The increased demand for spectrally efficient wireless communication chips necessitates the use of high linearity RF (Radio Frequency) front ends. Among other things, the high power amplifier in the transmitter is a crucial element which determines the overall linearity of the transmitted RF signal. Power Amplifiers exhibit crucial distortion effects at high power, namely amplitude and phase distortions. These are characterized as amplitude to amplitude modulation (AM-AM) and amplitude to phase modulation (AM-PM). Moreover the increasing popularity of Orthogonal Frequency Division Multiplexing (OFDM) as the choice of modulation has increased the linearity requirements of these PAs significantly, making AM-AM and AM-PM even more critical.
AM-AM distortion or gain compression causes intermodulation distortion (IMD) resulting in the folding of the out of band signal onto the desired signal spectrum, thereby resulting in higher bit error rate. AM-PM distortion or phase distortion causes unequal rotation in the received constellation causing difficulties in signal detection. The sum effect of AM-AM and AM-PM causes bit errors at the received signal as well as out of hand interference in the transmitted signal leading to violation of the defined FCC transmit spectral mask.
There have been a number of techniques which deal with modeling and compensation of AM-AM and AM-PM effects. These range from simple polynomial based models to complex models based on volterra series to model memory effects in PAs. Amplitude distortion effects (AM-AM) are considerably less challenging to measure. Among other techniques, a simple technique such as placing a power detector at the PA output and measuring the output power level while sweeping the power of the input sine test signal can be used to measure gain compression. However accurate measurements of AM-PM effects require high precision instrumentation, and hence, are expensive and time consuming. A common form of measuring AM-PM is using a Vector Network Analyzer (VNA). The measured phase of S2I parameter in the swept power S parameter measurement provides the AM-PM information of the amplifier. Besides cost and time (setup time and calibration time) considerations in this particular setup, the measurement is highly sensitive to calibration.
Hence, low cost test method for measuring AM-AM and AM-PM is desired for production testing solutions.